I use gravel for my bichirs, I hate sand. Sand would be fine to use, the only kind of bichir I had problems with with gravel were ornates I had one that would eat a mouth full of rocks with one pellet and spit out the rocks and if you ran by the tank when he was spitting them out I could see him getting scared and swallowing a stone. They are pretty good about not eating stones though, stress and taking off would be a more likely way of swallowing a stone than mistaking it for food.